Electric servomotor system



April 5, 1949. R D, MCCOY' 2,466,035

ELECTRIC SERVOMOTOR SYSTEM Filed Feb. 28, 1947 0 E MDUL TOR INVENTOR. m/EqwLEY D. McCoy ,Lg/1T TURA/EY.

Patented Apr. 5, 1949 UNITED sTATEs PATENT OFFICE ELECTRIC SERVOMOTORSYSTEM Rawley D. McCoy, Bronxville, N. Y., assignor to The SperryCorporation, a corporation of Dela- Application February 28, 1947,Serial No. 731,469

6 Claims.

This invention relates to servomotor systems, and, in particular, toimprovements in servomotor systems of the type having speed si-gnalgenerating means for damping or stabilizing servomotor operation.

In servomotor systems'that are employed to move or rotate a remoteobject in response to an error signal emanating from a reference device,objectionable hunting of the servomotor is encountered. To overcome thisobjectionable aspect of servomotor systems, Riggs, in U. S. Patent No.2,115,086 incorporates an A. C. eddycurrent generator also sometimesknown as a dynamic transformer, driven by the servomotor and productiveof an A. C. signal voltage of a phase dependent on the direction ofrotation and of an amplitude that is proportional to the speed of theservomotor. By applying this speed signal in a sense opposing the errorsignal from the reference device, the servomotor is thereby stabilizedand objectionable hunting prevented. However, in the system disclosed byRiggs, lag errors appear that are proportional to the output speed ofthe servomotor.

It is, therefore, the primary object of this invention to provide animproved method for damping servomotor operation while simultaneouslyreducing or eliminating speed lag.

Another object of this invention is to provide a servo motor systemhaving an A. C. speed voltage, or an eddy-current generator, with meansfor overcoming lag characteristics that are due to the degenerativeconnection of the speed signal. More particularly, the means hereinprovided to overcome inherent lag characteristics, include electricalcircuit means as distinguished from mechanical means such as aredisclosed, for example, in the application of Raymond C- Goertz,application Serial No. 608,052, led July 31, 1945..

It is a further object of the instant invention to provide means in aservo system, for producing a feed-back voltage which is combined in adegenerative sense with an error signal to control the servomotor, whichfeed-back voltage is substantlally reduced to zero under a steady stateor constant speed condition of the servomotor, while increasingoscillatory conditions or periods of acceleration or deceleration of theservomotor.

A still further object of the instant invention is to provide aservomotor system with an A. C. speed signal, which signal is combinedwith the error signal in a degenerative sense, and a D. C. speed signal,which signal is combined with the error signal in a regenerative sense.

The invention also relates to the novel features or principles of theinstrumentalities described herein, whether or not such are used for thes'tizated objects, or in the stated llelds or combinaons.

In the drawing like numerals refer to similar parts throughout theseveral views of which:

Fig. 1 illustrates a block diagram of the instant invention; and

Fig. 2 illustrates details of the component parts of Fig. 1.

Referring now to Fig. l of the drawing, a reference device, which may beof any of the many well-known types, illustrated as including thereference device I, having operatively associated therewith the rotor 2of a conventional synchrotransformer 3 which may be of the Selsyn,Telegon, or Autosyn type well known in the art. The rotor winding 2 isenergized by a source oi alternating current Il, and induces a voltagein stator winding 5 which is connected to a corresponding winding 6 of asecond transformer 'l having a rotor winding 3 positioned by shafts 9and I0 according to the position of the member 20 controlled by theservomotor I5. This reference device, productive of an error signal orcontrol voltage which'appears across the lines I I and this errorsignal, except in the manner modiiied as hereinafter set forth, istransmitted through a mixer demodulator I2 to a power amplifier I3 whoseoutput serves to excite winding' I4 of the servomotor I5, therebycausing the servomotor to be driven in accordance with the error signalthat appeared across II. The other winding of servomotor l5 is energizedfrom an alternating current source I6. Directly associated with theservomotor I5 by means of the driving shaft I1, is an eddy-currentgenerator i8. Generator I8 is energized by alternating current IB, andis productive of a speed signal or speed voltage in the winding I9,which signal varies in accordance with the, speed of the generator i8,and by virtue of the association of generator I8 with servomotor I5through shaft Il. The voltage generated in the winding I9 isproportional to servomotor speed and has a phase sense dependent uponthe direction of rotation. The voltage that is generated in winding I9,hereinafter known as the speed voltage, is transmitted by lines 2l and22 to mix with the error signal received by the system on the windings II, but in an opposing or degenerative sense. Where the system is afollow-up system, the servomotor operating to drive an object insubstantial synchronlsm with a reference, this speed voltage produces anincreased constant speed condition is provided.

lag between the object and the reference due t the degenerative effectof the speed voltage.

In the circuit thus far considered, a considerable lag is createdbetween the error signal and servomotor operation. To overcome thishandicap, the speed signal generated in winding I9 is transmitted alonglines 2| and supplied to the linear demodulator 23. This demodulator 23receives the alternating current speed voltage, demodulates it tosubstantially unidirectional voltage, whereupon the demodulated speedvoltage is transmitted through the resistance-capacitance circuit 23,including resistance 28 and the capacitance 28. From thisresistance-capacitance network the demodulated speed signal is appliedto the mixer demodulator I2. As the mixer demodulator I2 (to be laterexplained in more detail) has an equalv gain for A. C. and D. C.

signal or the D. C. speed signal will appear to augment the error signalreceived from lines II, thereby overcoming any lag -tendencies thatthesystem would otherwise have.

As the frequency of oscillation increases, the

l effectiveness of the condenser 28, insofar as proinput signals and,inasmuch as the demodulated speed signal is delayed in time by theresistancecapacitance network 24 and then inserted into the mixer I2 ina sense aiding the error signal I2, or in a regenerative sense, speedlag is effectively overcome during steady state operation. Morespecically, during a steady state condition of servomotor operation, thedemodulated speed signal, after passing through theresistance-capacitance network 24, becomes substantially equal to thespeed signal that is supplied along lines 22.

'As the speed signal from the lines 22 is placed in opposition to theerror signal and as the demodulated and time-delayed speed signal fromnetwork 24 is entered into the demodulator I2 in a sense aiding theerror signal, the net result during a steady state servomotor conditionwill be that the error voltage II alone will be eiective to governservomotor operation and speed lag will be effectively eliminated.During oscillatory conditions including acceleration or deceleration ofthe servomotor I5, in response to similar error signal variations, thenetwork 24 and particularlyv the condenser 26 loses its infiniteimpedance and in conjunction with the resistance 25, serves as a phaseshifting network for the demodulated speed signal. As the frequency ofoscillation increases the signal output from the network 24 willdecrease and the eilectiveness of the speed signal 22 in dampingservomotor operation will become greater.

Thus, for given error signal productive of a steady servomotor speed, apair of speed signals will be generated one being an A. C. signal andthe other being a time-delayed D. C.,signal. As these speed signals areinserted into an amplifier having equal gain for A. C. and D, C. inputsignals, and further, asV the A. C. signal is inserted in the amplifierin a degenerative sense, and the D. C. signal is inserted in aregenerative sense, their net eect will be to provide lag free operationduring the steady state condition of the servo system. Duringthis-steadystate condition, the error signal alone `will be operative inproducing servomotor displacement.- In this manner, va system completelyfree of speedlag durin During periods ofY oscillation suchnas wouldviding an impedance is concerned, is greatly decreased compared with thesteady state condition, and thusly, the D. C. speed-signal that opposesdamping during a steady state, is diminished, allowing the A. C. speedsignal appearing along lines 22 to have a greater eilfect insofar asdamping is concerned. Thus, as the servomotor speed decelerates orapproaches zero, the A. C. speed signal from lines 22 is very effectivein providing the desired speed damping.

'I'he diagram of Fig. 2 is similar in all major respects to the diagramin Fig.y l, excepting insofar as Fig. 2 illustrates preferable internalcomponents of several of the elements that were illustrated in blockform in Fig. 1. 1

The mixer demodulator I2 is illustrated as comprising a transformer 21for receiving the combined error signal and what may be termed the A. C.speed signal (that is, the speed signal transmitted along lines 22). TheD. C. speed signal is received from the demodulator 23, which is ofconventional design and may include input transformer 43, rectifyingtubes 44 and 4-5 and transformer 46 which is energized by A. C. sourcei6. The resistor capacitance network 24 then receives the demodulated orD. C. speed signal whereupon it is inserted into the mixer demodulatorI2 on the secondary windingsof the input transformer 21. The remainderof this balanced demodulator I2 is of a design well known to the art andincludes resistors 28 and 29 and the triodes 3| and 32, these triodeshaving a D. C. grid bias 33. The demodulator l2 is productive of alinear output which appears across the plate circuits of the tubes 3|and 32 and is transmitted therefrom as an input to the power amplifierI3. The unidirectional signal voltage appearing between point 33 and themidpoint 34 in the output of the demodulator `or between point 35 andthe midpoint 34 will provide a differential voltage across 33 and 35 of'a magnitude and polarity dependent respectively upon the amplitude andphase sense of the error signal across lines II.

An A. C. source I8 of reference voltage, preferably the same as thatfrom which the primary control appearing across lines `Il is derived,feeds the two primary windings of the transformers 36 and thesecondaries thereof supply plate potentials to the plates of tubes 3|and 32. The unidirectional voltage from the demodulator I2 is 39.lTransformer 4| supplies plate voltageto the plates of tubes 38 and 39.The rplate circuits of tubes 38 and 39 are connected across the occur ifthe error signal vwere oscillating or was,

deceleration of the servomotor I5, the immediate eiIect of the outputfrom'the: eddy-current generatorgl would be to increase theA. C. speedsignal component appearing on line 22 vvthereby causing speed vdampingand simultaneously, after changing-in a sense Ato causean acceleration vor f D. C. speed signal.

' a slight delay (caused by the condenser-resistor center-tapped primaryof coupling transformer 42,` and the secondary thereof is vconnected to.energize one phase of the servomotor I5, herein represented forexemplary purposes as a twophase motor. The`servomotoris thereforedriven in accordance with the error signal opposed by lthe A. C. speedsignal which in turn is opposed,

but with a time delay, by the 'demodulated or Since many changes couldbe made in the above commotion and many apparently widely dinerentembodiments of this invention could be made 5 without departing from thescope thereof, it is intended that all matter contained in the abovedescription or shown in the accompanying drawings shall be interpretedas illustrative and not in a limiting sense.

What is claimed is: l

l. In a servomotor system, a motor, a source of control signal voltage,amplier means for controlling the rate and direction of operation ofsaid motor in accordance with the magnitude and polarity sense of saidsignal, an alternating current speed generator driven by said motor forsupplying an alternating voltage having an amplitude proportional to thespeed of the motor and of a phase sense dependent upon the direction ofmotor operation, a feed-back circuit for applying said speed voltage indegenerative fashion to said amplifier, and a second feed-back circuitconnected to receive said speed voltage and including means for delayingthe .transmission of signal voltages therethrough, said second feed-backcircuit being connected to apply the delayed signal voltage inregenerative fashion to said amplier.

2. In a servomotor system, a motor, a source of control signal voltage,amplifier means for controlling the operation of said motor inaccordance with said signal, generator means operatively associated withsaid motor and having an output voltage proportional to motor speed,circuit means for applying said speed voltage in degenerative fashion tosaid ampliiler, and means connected to receive said speed voltageincluding means for delaying the transmission of signal voltagestherethrough, said last-mentioned means being arranged to apply saiddelayed signal volttage in a regenerative fashion to said amplifier.

3. In a servomotor system, a motor, a source of control signal voltage,amplifier means for controlling the operation of said motor inaccordance with said signal, generator means operatively associated withsaid motor and having an alternating current output voltage proportionalto motor speed. circuit means for applying said alternating currentspeed voltage in a degenerative fashion to said ampliner, anddemodulator means connected to receive and dei modulate said speedvoltage including resistancecapacitance circuit means for delaying thetransmission of signal voltages therethrough, said demodulator meansbeing connected to apply said delayed and demodulated signal voltage ina regenerative fashion to said ampliiier.

aandoen 4. In a servomotor system, a motor, a source of control signalvoltage, amplier means having an equal gain for alternating current anddirect current input for controlling the operation of said motor inaccordance with said signal, generator means operatively associated withsaid motor and having an alternating current output voltage proportionalto motor speed, circuit means for applying said alternating currentspeed voltage in degenerative fashion to said amplifier, and demodulatormeans connected to receive said speed voltage and including means fordelaying the transmission of signal voltages therethrough, saiddemodulator means being connected to apply said delayed and demodulatedsignal voltage in a regenerative iashion to said amplifier.

5. In a servomotor system, a servomotor, an

error signal source, and means for damping servomotor operation, meansfor preventing lag between said servomotor and said error signalcomprising generating means having an alternating current output voltageresponsive to servomotor speed, demodulating means for receiving saidspeed responsive voltage, means connected to receive and delay saiddemodulated speed voltage, and circuit means connecting said delayed anddemodulated speed voltage in a regenerative sense with said errorsignal.

6. In a servomotor system, an error signal source, a servomotoroperatively connected to said error signal source to be driven inresponse thereto, signal generating means for producing an outputvoltage in accordance with servomotor A speed, circuit means connectingsaid output voltage in a degenerative sense with said error signal, asecond circuit means connected to receive said output voltage andincluding means for delaying the passage of voltages therethrough, saiddelayed voltages being connected in a regenerative sense with said errorsignal.

RAWLEY D. McCOY.

REFERENCES CITED UNITED STATES PATENTS Name Date Jones June 20, 1944Number

